A dry sump is a lubricating oil management strategy for four-stroke and large two-stroke piston internal combustion engines that uses an external secondary reservoir for oil, as compared to a conventional wet sump system.
Four stroke engines, for example, are lubricated by oil which is pumped into various bearings and thereafter allowed to drain to the base of the engine. In most production automobiles, for example, which use a wet sump system, this oil is simply collected in a three to seven litre capacity pan at the base of the engine, known as the oil pan. From there it is pumped back up to the bearings by the oil pump, which is typically internal to the engine.
In a dry sump engine the oil also falls to the base of the engine, however, rather than collecting in an oil pan, the oil is pumped into another external reservoir by one or more suction (scavenger) pumps. Oil is then pumped from this external reservoir to the bearings of the engine by a pressure pump.
Having a dry sump lubrication system provides several advantages over wet sump systems, including, for example, increased oil capacity, decreased parasitic loss and a lower center of gravity for the engine. Because the reservoir is external to the engine, the oil pan can be much smaller in a dry sump system (as compared to a wet sump system), allowing the engine to be placed lower in a vehicle. In addition, the external reservoir can be as large as desired, which is not the case in a wet sump system as the more oil capacity increases, the larger the oil pan. Larger oil pans raise the engine even further. Furthermore, increased oil capacity by using a larger reservoir typically leads to cooler oil. In addition, dry sump designs are not susceptible to the oil starvation problems wet sump systems suffer from if the oil sloshes in the oil pan temporarily uncovering the oil pump pickup tube. Finally, having the pumps external to the engine allows them to be maintained or replaced more easily, as well.
Dry sump engines, are, however, not without their drawbacks. On the downside, it is generally difficult to withdraw oil from the lubrication system of a dry sump engine—e.g. for maintenance purposes (to change the oil)—as the oil does not all simply collect in an oil pan by gravity. What typically occurs in dry sump engines is that when the engine is not in operation, gravity causes the oil to collect at various points throughout the engine. Therefore, in order allow the engine oil to be changed, designers of such engines place several oil drain plugs or access openings in proximity to the various positions throughout the engine where oil will collect. A person desirous of changing the oil (be they an end-user or a repair person) must remove all of the plugs and use all of the access openings to drain the oil. It is known, however, that not all such persons want to go to the effort of opening multiple oil plugs and/or of using auxiliary draining devices (e.g. suction tubes) to drain the oil from the engine. Even when they do want to do so, sometimes they forget to use all of the plugs and/or openings. As a result, in many cases a significant amount of used oil remains in the system after an oil change of a dry sump. This leads to inferior quality of the oil and increased wear of engine components.
There is a need for an lubrication system for a dry sump engine that addresses at least this concern.